This invention relates to a type of controlled release capsules, i.e. capsules with permeable or semi-permeable walls which have the ability to release a solute or a volatile liquid or the like at a predetermined rate. The release of solute is an effect resembling osmosis, while the release of the volatile liquid is an effect produced by partial pressure differences across the capsule walls. An aspect of this invention relates to a process for making the capsules.
There is considerable utility in the art of encapsulation for a means which permits the release of an encapsulated chemical over a predetermined period of time. For example, it is desirable to make one application of agricultural chemicals (fertilizers, pesticides, herbicides, etc.) which is effective for a complete growing season, rather than several successive applications. Also, it is desirable to apply only the necessary amount of fertilizer, thereby preventing ground water pollution.
Various types of coatings (U.S. Pat. No. 3,223,518) and porous packaging films (U.S. Pat. No. 3,059,379) have been suggested for encapsulating solid agricultural chemicals. However, the coatings cannot be utilized to contain aqueous solutions, while the flexible packaging films are not practical for encapsulating liquids. U.S. Pat. No. 2,791,496 discloses the impregnation of exfoliated vermiculite with liquid agricultural chemicals, but this product does not provide a controlled release rate. U.S. Pat. No. 3,423,489 discloses crystalline polyolefin capsules for encapsulating liquids, but these capsules are not suitable for controlled release of the contained liquid.
Accordingly, this invention contemplates providing liquids such as agricultural chemicals in a form which permits release of the chemical over a predetermined period of time. This invention also contemplates a means and method for introducing a controlled amount of microporosity into a membrane which serves as the wall or shell of a capsule.